CUB-228: add battery_calibration_offset to cameras table

- Add column to 001_create_tables.sql for fresh databases
- Add migration 002 for existing databases (idempotent via
  pragma_table_info check)
- Implement runIncrementalMigrations in db.go
- Add BatteryCalibrationOffset to Camera model
- Update all camera SELECT queries (cameras List, detail, MQTT
  subscriber getCamera, register)

internal/mqtt/subscriber.go

@@ -404,9 +404,9 @@ func extractCameraID(topic string) string {
 func getCamera(db *db.DB, cameraID string) (models.Camera, error) {
 	var cam models.Camera
 	err := db.QueryRow(
-		"SELECT camera_id, friendly_name, COALESCE(mac_address,''), created_at, updated_at FROM cameras WHERE camera_id = ?",
+		"SELECT camera_id, friendly_name, COALESCE(mac_address,''), COALESCE(battery_calibration_offset, NULL), created_at, updated_at FROM cameras WHERE camera_id = ?",
 		cameraID,
-	).Scan(&cam.CameraID, &cam.FriendlyName, &cam.MacAddress, &cam.CreatedAt, &cam.UpdatedAt)
+	).Scan(&cam.CameraID, &cam.FriendlyName, &cam.MacAddress, &cam.BatteryCalibrationOffset, &cam.CreatedAt, &cam.UpdatedAt)
 	return cam, err
 }